Brachytherapy is a form of radiotherapy where a source of ionising radiation is placed inside or next to the area requiring treatment. It usually involves placing radionuclides in or close to a target in order to deliver radiotherapy. These can be injected into the patient and left in place permanently while the radioactivity decays, or removed after treatment is complete.
A variant of this is so-called high dose rate (“HDR”) brachytherapy. This is administered by inserting one or more catheters into the body to pre-defined (planned) positions in the body. A radioactive source is then propelled from a shielded container along to the end of each catheter for a predetermined time, and then withdrawn back into the shielded container. While the source is inside the patient, it delivers the therapeutic dose to the target. This will inevitably also deliver a dose to the surrounding tissue.
One particular application of HDR is referred to as “Accelerated Partial Breast Irradiation” (“APBI”) in which a small, single breast tumour is surgically removed together with a “margin” of tissue beyond the tumour, leaving a cavity in the breast. To prevent tumour recurrence caused by any remaining microscopic malignant or pre-malignant cells in the tissue surrounding this cavity, a course of radiation therapy is applied to that tissue. In conventional radiation treatment, radiation is applied by external beam radiotherapy once a day for typically 5-8 weeks following surgery. With APBI using HDR, the treatment regime is accelerated to (typically) two treatments a day for just five days.
In one type of HDR treatment, a single Iridium-192 (Ir-192) HDR source is used to treat the breast. Either during or a few days after surgery, an applicator comprising a catheter surrounded at its distal end by an inflatable balloon is inserted into the cavity in the breast under local anaesthetic, and water is injected to inflate the balloon so that it fits snugly inside the cavity. The most well-known balloon applicator is called MammoSite, made by Hologic Corp. At each treatment session an HDR radioactive source is passed from its shielded container down the central catheter to specific positions in the balloon for specific periods of time, governed by the requirements of the treatment plan, to deliver the therapeutic radiation. The source is then completely withdrawn back into its shielded container. Typically the source is in place for between 5 and 20 minutes for each treatment session, depending on the age (and hence radioactivity) of the radionuclide and the requirements of the treatment plan. The inflated applicator remains in place within the breast until the final treatment session has finished, whereupon it is deflated and removed.
Recently a form of APBI treatment has been developed using a miniature X-ray tube instead of a radionuclide (the Xoft Axxent system). We will refer to this as “Electronic HDR” (“E-HDR”). The advantage of using an X-ray tube is that the radiation can be turned off when it is not needed, so it has considerable safety and convenience factors compared with conventional radionuclide based HDR. Xoft Inc and others have disclosed the concept of varying the tube voltage (and hence the energy spectrum of the emitted x-rays) and/or the current (and hence the dose rate) and/or the direction of the x-ray beam in such a way as to shape the distribution of the therapeutic radiation dose in order to minimise the dose to unaffected regions or critical organs.
The field of brachytherapy is distinct from that of external beam radiation therapy, which uses linear accelerators to deliver radiation from outside the body. In external beam radiation therapy various techniques have been developed for directing, collimating and operating the treatment beam (usually to turn it on and off) based on the path of the beam relative to the target and other regions of the patient, and the state of the patient including their breathing cycle. The aim of these has been to maximise the dose applied to the target region while minimising the dose to non-target regions and (in particular) to sensitive regions. These techniques include interventional methods which arrest breathing for certain periods (such as the Elekta Active Breathing Coordinator), and free breathing methods which measure lung volume, target position, or assumed surrogates for these and control the beam based on acceptance limits (eg the Varian RPM and Brainlab Exactrac systems).